Micro- and nanotechnologies are at the centre of numerous investigations and huge investments, particularly in the areas of information technologies, food and health care. Coupling the skills in mineral synthesis, organic synthesis, physical-chemistry of complex media, and physics of materials has been the key for the success of these developments. Nano and micro particles in particular have a wide range of industrial applications such as in healthcare, medical, and photographic emulsions. In other applications, nano/micro particles have been used as nucleation centers, which may be used to form larger particles with specific constructions.
Silver has long been considered a powerful and natural antibiotic and antibacterial. The combination of silver and nano/micro particles is extremely attractive in many areas. The extremely small size of silver nano and micro particles means they exhibit high surface to volume ratios, and thus enhanced surface related properties such as catalysis when compared with bulk silver. This allows them to easily and aggressively interact with the environment, including microorganisms as well as other environmental agents, thus increasing their antibacterial efficiency.
In addition to having an antibacterial effect, silver has antifungal and deodorizing effects that have been recently exploited commercially. A study from the University of Texas and Mexico University was recently published in the journal Nanotechnology that showed silver nanoparticles are able to kill HIV-1. This study looked at the effect of silver nanoparticles in the range of 1-100 nm on Gram-negative bacteria using high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM). Their results indicated that the bactericidal properties of the nanoparticles were size dependent, since the only nanoparticles that presented a direct interaction with the bacteria preferentially have a diameter of ˜1-10 nm. The authors have postulated that the nanoparticles could kill other viruses in addition to HIV-1. The benefits of silver nanoparticles continue to be explored. Researchers are optimistic that nanoengineered silver may be the solution to controlling many types of viruses. Silver may now come under consideration as an alternative to drugs when it comes to fighting previously untreatable viruses such as the Tamiflu-resistant Avian flu.
Worldwide there is a very significant economic loss in discarded foods due to spoilage. For example, one source reports that the food industry annually discards $35 billion worth of spoiled goods. Fifty-six percent of all supermarket store shrink is reported to come from perishables, according to a 2003 survey. Currently the trend in the food growing and retail industry is to engineer the sustainable supply chain. This entails extending and measuring the usable life of fruits and other produce as well the process of saving energy required for continuously cooling fruits and other produce to preserve the freshness by keeping the bacterial count low. Synergistically, industry trends are towards an increase in healthier lifestyles by consuming more fresh-cut produce with a demand for eco-friendly sustainable packaging that can sense, monitor, communicate and extend the useable life of foods. In particular, current methods used for extending the useable life of these products include pesticides, fertilizers and picking the produce before it is ripe. Other methods include various packaging alternatives, which include control of respiration and the depleting of area ethylene.
Various processes to produce nano/micro particles are known. For example, U.S. Pat. No. 7,128,816 issued to Denes, et al. discloses a process for producing colloidal dispersions of nanoparticles of electrically conducting materials. The colloidal dispersions are produced in a dense media plasma reactor that has at least one static electrode and at least one rotating electrode. Minute particles are sputtered off of the electrically conducting material from which the electrodes are made.
Indian patent 192012 describes an emulsion process for preparing porous polymer nanoparticles.
International patent application publication WO9106036 discloses methods of coating a nanoparticle with one or more layers of various types of materials. It also discloses a method for preparation of metal-coated nanoparticles, in which the metal halide nanoparticles are prepared and exposed to ultraviolet light to change the metal halide to metal to form metal coatings over individual nanoparticles. In another process variant, silver-coated particles are prepared by a process by providing silver ion source and halide ion source to produce silver halide coated nanoparticles and subsequently exposing the silver halide nanoparticles to ultraviolet light in EDTA to reduce the silver halide coating to silver-coated nanoparticles. In a further process variant, silver halide coated particles and an electron scavenger are contained in an anaerobic liquid carrier and uniformly distributed therein, such that exposing the liquid carrier to light of sufficient strength and for a sufficient time reduces the silver halide coating to metallic silver.
In Cong et al., “Hollow Cu—NP Spheres Made from Electroless Cu Deposition with Colloidal Particles as Templates,” a process is described for producing hollow copper spheres in the nanoparticle range using SiO2 and PSMA nanoparticles as a template core. The core is removed to yield a hollow metal nanoparticle. Wall thickness on the SiO2-produced particles was measured to be about 30 nm, and wall thickness on the PSMA-produced particles was measured to be about 55 nm, although the thinner-walled SiO2-produced particles were less likely to break during core removal.
International patent application publication WO9106036 discloses methods of coating a nanoparticle with one or more layers of various types of materials. It also discloses a method for preparation of metal-coated nanoparticles, in which the metal halide nanoparticles are prepared and exposed to ultraviolet light to change the metal halide to metal to form metal coatings over individual nanoparticles. In another process variant, silver-coated particles are prepared by a process by providing silver ion source and halide ion source to produce silver halide coated nanoparticles and subsequently exposing the silver halide nanoparticles to ultraviolet light in EDTA to reduce the silver halide coating to silver-coated nanoparticles. In a further process variant, silver halide coated particles and an electron scavenger are contained in an anaerobic liquid carrier and uniformly distributed therein, such that exposing the liquid carrier to light of sufficient strength and for a sufficient time reduces the silver halide coating to metallic silver.
All of the processes discussed in the above application for preparing silver-coated nanoparticles are tedious and lengthy. They are also limited in that they could only produce coated nanoparticles having a coat of a certain size or greater. Silver bromide is insoluble in water, the medium that is being used in these processes for the deposition from a silver nitrate or a soluble silver salt. It requires a very high gelatin concentration to hold the particle size to the nanoparticle range, which is a very well known method of producing high-speed photographic films. If the gelatin concentration is low, and the silver concentration is high, say about 1%, then the bromide precipitates in a very coarse particle size. Thus if one were depositing a silver film, the result would be the deposit of silver particles of almost matching dimensions, and therefore a mixture of nanoparticles and large particles.
The processes discussed in the above applications for preparing coated silver nano/micro particles are tedious and lengthy. It would be desirable to provide an efficient, less tedious method of coating the nano/micro particles that also increases the effective surface area of the silver several-fold, allowing reduction of total used silver and production in a shorter amount of time.